First, classification of NAT (network address translation) is explained. The NAT includes the transmission port assigning rule and reception filter rule, and the NAT is classified according to their combination. The transmission port assigning rule is divided into a cone type in which, not depending on the packet destination (IP address, port), when the port of NAT local side (for example, LAN side) information processor and the IP address are the same, the port of the global side (for example, the Internet or WAN side) assigned to the NAT is the same, an address sensitive type in which a new port is assigned for each packet destination address, and a port sensitive in which a new port is assigned for each destination port of packet. The reception filter rule for judging possibility of reception of packet from the local side to the port transmitting the packet from the NAT local side is divided into an address sensitive filter for receiving the packet only from the address transmitting the packet from the port, a port sensitive filter for receiving the packet only from the port transmitting the packet from the port, and no filter having no filter. By combining the transmission port assigning rule and reception filter rule, the NAT can be classified into the following nine types (see FIG. 32).
Full cone NAT (F NAT): Transmission port assigning rule of cone type, and reception port filter rule of no filter.
Restricted cone NAT (R NAT): Transmission port assigning rule of cone type, and reception port filter rule of address sensitive filter.
Port restricted cone NAT (PR NAT): Transmission port assigning rule of cone type, and reception port filter rule of port sensitive filter.
Symmetric (a) NAT (Sa NAT): Transmission port assigning rule of address sensitive type, and reception port filter rule of no filter.
Address sensitive symmetric NAT (AS NAT or Sb NAT): Transmission port assigning rule of address sensitive type, and reception port filter rule of address sensitive filter.
Symmetric (c) NAT (Sc NAT): Transmission port assigning rule of address sensitive type, and reception port filter rule of address sensitive type filter.
Symmetric (d) NAT (Sd NAT): Transmission port assigning rule of port sensitive type, and reception port filter rule of port sensitive type filter.
Symmetric (e) NAT (Se NAT): Transmission port assigning rule of port sensitive type, and reception port filter rule of address sensitive type filter.
Port sensitive symmetric NAT (PS NAT or Sf NAT): Transmission port assigning rule of port sensitive type, and reception port filter rule of port sensitive filter.
In the NAT communication, it has been proposed to establish communications between PC1 and PC2 without using server as shown in FIG. 33.
This technology is disclosed, for example, by D. Yon (Connection-Oriented Media Transport in SDP, Online, March 2003, [searched Sep. 29, 2003], Internet <URL: http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdp-comedia-05.txt>, hereinafter called non-patent reference 1), or Y. Takeda (Symmetric NAT Traversal using STUN, Online, June 2003, [searched Mar. 25, 2004], Internet <URL: http://www.cs.cornell.edu/projects/stunt/draft-takeda-nat-traversal-00.txt>, hereinafter called non-patent reference 2). It is also reported by J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy (STUN—Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs), Online, March 2003, Network Working Group Request for Comments: 3489, [searched Mar. 24, 2004], Internet <URL: http://www.ietf.org/rfc/rfc3489.txt>, hereinafter called non-patent reference 3).
In this case, probably, there is a combination of NATs unable to establish communications. In FIG. 40, when communicating from PC1 of information processor connected to NAT1 local side to PC2, let us call NAT1 as sending side NAT, and NAT2 as receiving side NAT. Hence, combination of NATs capable of establishing communications between PC1 and PC2 is as shown in FIG. 34.
In FIG. 34, connection of *1 has been known hitherto, connection of *2 is disclosed in non-patent reference 1, and connection of *3 is disclosed in non-patent reference 2. Even in the combination of NATs capable of establishing communications, connection of *3 involves an uncertainty because communications cannot be established unless the port number differential of NAT is known and the position of the latest port of receiving side NAT is known.
In the combinations in FIG. 34, cases of using Sa NAT or Sc to Se NAT are not included, but even in such NAT cases, it is desired to establish peer-to-peer communication by way of NAT (for example, communication between PC1 and PC2 in FIG. 33).
As prior conditions for establishing communications between information processors, it is desired to detect securely the position of ports of NAT passing through the bubble packet (the packet sent in order to leave communication record in the NAT) transmitted from one information processor.